What is the Milky Way?

by Fraser Cain on November 4, 2013

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The Earth orbits the Sun in the Solar System, and the Solar System is embedded within a vast galaxy of stars. Just one in hundreds of billions of galaxies in the Universe. Ours is called the Milky Way because the disk of the galaxy runs across the sky as a band of glowing light, like spilled milk.

Astronomers had suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many many more stars in the sky, and all of the stars that we can see are a part of the Milky Way.

If you could travel outside the galaxy and look down on it from above, you’d see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have 2 spiral arms: they are called Scutum–Centaurus and Carina–Sagittarius.

Artist's conception of the Milky Way galaxy. Credit: Nick Risinger

Artist’s conception of the Milky Way galaxy. Credit: Nick Risinger

It’s very difficult to figure out what the Milky Way looks like exactly, because we’re embedded inside it. If you’d never been out of your house, you wouldn’t know what it looked like from outside. But you’d get a sense by looking at other houses in your neighborhood.

Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way. The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region.

Astronomers estimate that there are between 100 and 400 billion stars in the Milky Way, and think that each star has at least one planet. So there are likely hundreds of billions of planets in the Milky Way, and at least 17 billion of those are the size and mass of the Earth.

Our Sun is located about 27,000 light years from the galactic core. At the heart of the Milky Way is a supermassive black hole, just like all of the other galaxies. This monster is more than 4 million times the mass of the Sun.

This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech

This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech

Our Sun takes about 240 million years to orbit the Milky Way once. Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made 18-20 trips around in its entire life.

The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter. Nobody knows what it is, but its mass helps keep the galaxy from tearing itself apart as it rotates.

It’s believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with Andromeda in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their supermassive black holes might even merge.

The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster.

Dung Beetle. Credit - Kay-aftrica

Dung Beetle. Credit – Kay-aftrica

You might be amazed to know that dung beetles actually navigate at night using the Milky Way. If you’ve never seen the Milky Way with your own eyes, you should take the chance when you can. Go to a place with nice dark skies, free from light pollution. Look up and appreciate the Milky Way, and wave hello to all the neighboring stars who share our galaxy with us.


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Kawarthajon November 4, 2013 at 11:54 AM

What causes the “density waves” in the Milky Way?

IVAN3MAN_AT_LARGE November 4, 2013 at 6:37 PM

Wikipedia has an article on it: Density wave theory.

Shootist November 4, 2013 at 12:50 PM
JonHanford November 5, 2013 at 9:32 AM

I see a description of the galactic disk (and mention of the dark matter halo) but no mention of the other major components of the Milky Way, namely the galactic halo and bulge:



Felix Kwan November 5, 2013 at 11:27 PM

like to learn more the complexity of space and time. as we look in the night sky thousand stars, some r thousand and thousand of years the light reach, is it we looking back in time, can we travel back in time?

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